Transparent substrate comprising a stack of thin layers for electromagnetic armour

ABSTRACT

A transparent substrate, especially made of glass, provided with a thin-film multilayer ( 20 ) that includes three silver layers (Ag 1,  Ag 2,  Ag 3 ) and comprises, alternately on the substrate, a titanium dioxide layer ( 21 ), a metal oxide layer ( 22 ), one of the silver layers (Ag 1,  Ag 2,  Ag 3 ) and a covering layer ( 23 ), characterized in that: the metal oxide is zinc oxide; 
         the covering layer ( 23 ) is a sacrificial metal; and    an antireflection layer ( 24 ) comprising at least one metal oxide is deposited on the covering layer ( 23 ) for the silver layer (Ag 3 ) furthest away from the substrate.

The subject of the invention is a transparent substrate, especially madeof glass, which is coated with a thin-film multilayer, comprising atleast one metal layer, for electromagnetic shielding.

The invention will be more particularly described for the use of such asubstrate in a plasma display screen; however, it is not limited to suchan application, it being possible for the substrate to be inserted intoany electromagnetic shielding wall.

A plasma display screen comprises a plasma gas mixture (Ne, Xe, Ar)trapped between two glass plates, and phosphors placed on the internalface of the rear plate of the display. Ultraviolet light radiationemitted by the plasma gas mixture during the plasma discharge betweenthe two glass plates interacts with the phosphors on the internal faceof the rear plate in order to produce the visible light radiation (red,green or blue). A gas particle deexcitation mechanism competes with theUV emission, which generates infrared radiation between 800 and 1250 nm,the propagation of which, mainly through the front face of the display,may be the source of very troublesome interference, especially asregards equipment located nearby and controlled by infrared, for exampleby means of remote controls.

Moreover, like all electronic apparatus, plasma display screens possessaddressing systems or drivers that may generate parasitic radiationwhich must not interfere with other devices, such as microcomputers,mobile telephones, etc.

To eliminate, or at the very least attenuate, the propagation of suchradiation, one solution consists in placing against the front face ofthe display a window, also called a filter, which is both transparentand metallized in order to provide electromagnetic shielding. Thisfilter is, for example, a transparent substrate coated with thinsilver-based layers that reflect the electromagnetic waves in thefrequency range from 30 MHz to 1 GHz and infrared beyond 800 nm.

Thus, patent FR 2 641 272 proposes a substrate comprising a reflectivesilver layer sandwiched between a transparent sublayer that comprises atleast one layer of a metal oxide, and a transparent covering layer thatcomprises a layer of sacrificial metal oxide, a zinc oxide layer, thethickness of which does not exceed 15 nm, and an upper covering layer ofa metal oxide.

The silver layer preferably has a thickness of between 8 and 12 nm.

The metal oxide layer of the sublayer may be chosen from several oxidesand may be a mixture of several oxides. A preferred example is atitanium dioxide layer and a tin oxide layer deposited on said titaniumdioxide.

The object of the sacrificial metal oxide is to protect the silver layerfrom oxidation, in particular during its deposition when this is carriedout by the technique of sputtering. This is because, if the silver wereto be impaired, the coated substrate would lose its low emissivity andits light transmission would be greatly reduced. The sacrificial metaloften preferred is titanium, as it provides the silver with veryeffective protection against oxidation and has the advantage of beingeasily oxidized, to form an oxide of very low absorbency.

The zinc oxide layer serves as protection against the penetration ofoxygen into the lower layers and allows the thickness of sacrificialmetal to be reduced somewhat, this metal then being more easily, morecompletely and more uniformly oxidized. The above document requires alimited thickness for the zinc oxide layer of 15 nm, in particular so asto give the layer good light transmission properties.

However, such a substrate with a single metal layer is not suitable forobtaining sufficient electromagnetic shielding, such as to have asurface resistance of less than 1.8 Ω/□. Furthermore, other patentapplications propose multilayers containing a plurality of metal layers,in particular silver layers. However, it is known that increasing thenumber of layers reduces the light transmission; a compromise betweenthicknesses and types of layer must therefore be found in order toachieve satisfactory light transmission.

The patent application published under WO 01/81262 proposes a multilayerhaving two silver layers, with a thickness e₁ in the case of the silverlayer closest to the substrate and a thickness e₂ for the other layer, asacrificial metal oxide, such as titanium oxide, being placed above eachsilver layer in order to protect it. One example of a sequence is thefollowing:

-   -   substrate/Si₃N₄/ZnO/Ag/Ti/Si₃N₄/ZnO/Ag/Ti/ZnO/Si₃N₄.

To achieve a surface resistance of less than 1.8 Ω/□, while stillmaintaining a suitable light transmission, the ratio of the thicknessese₁/e₂ is between 0.8 and 1.1, preferably between 0.9 and 1, and thetotal thickness of the metal layers, e₁+e₂, is between 27.5 and 30 nm,preferably between 28 and 29.5 nm.

European patent application EP 1 155 816 discloses a multilayer havingthree, or even four, silver layers with an alternation of a titaniumoxide layer and of a layer having a refractive index of less than 2.4 ata wavelength of 550 nm, such as for example zinc oxide or preferablysilica nitride. The thickness of the silver layer closest to thesubstrate and of that furthest away is preferably equal to 0.5 to 1times the thickness of the other silver layer. An example of a sequencehaving a surface resistance of 1.5 Ω/□, with a light transmission of67%, is given with three palladium-doped silver layers each having athickness of 16 nm. This sequence is the following:

-   -   substrate/TiO_(x)/SiN_(x)/Ag/SiN_(x)/TiO_(x)/SiN_(x)/Ag/SiN_(x)/TiO_(x)/SiN_(x)/Ag/SiN_(x)/TiO_(x).

However, it is always desirable for the properties of existing solutionsto be further improved, and thus obtain an even more substantialreduction in surface resistance without degrading the lighttransmission.

The object of the invention is therefore to find another filtersolution, especially for a plasma display screen, in order to alleviatethe problem of electromagnetic wave transmission, while still achievingsatisfactory optical properties.

According to the invention, the transparent substrate, especially madeof glass, provided with a thin-film multilayer that includes threesilver layers and comprises, alternately on the substrate, a titaniumdioxide layer, a metal oxide layer, one of the silver layers and acovering layer, characterized in that:

-   -   the metal oxide is zinc oxide;    -   the covering layer is a sacrificial metal; and    -   an antireflection layer comprising at least one metal oxide is        deposited on the covering layer for the silver layer furthest        away from the substrate.

According to one feature, the thickness of each of the silver layers isbetween 13 nm and 19 nm. The thicknesses (e_(Ag1), e_(Ag2), e_(Ag3)) ofthe three respective layers (Ag₁, Ag₂, Ag₃) are identical, or else theyvary in a ratio of between 0.8 and 1.2 and are such thate_(Ag1)≦e_(Ag3)≦e_(Ag2).

According to another feature, the titanium dioxide layer as sublayer forthe silver layer (Ag₁) closest to the substrate has a thickness ofbetween 10 and 20 nm, preferably between 10 and 15 nm, and the titaniumoxide layers as sublayers for the other two silver layers (Ag₂, Ag₃)have a thickness of between 35 and 55 nm, preferably between 40 and 50nm.

Preferably, the zinc oxide layer has a thickness of greater than 15 nm.

Advantageously, the sacrificial metal layer is of niobium, titanium orzirconium, and has a thickness not exceeding 2 nm.

According to another feature, the antireflection layer has a thicknessof between 25 and 50 nm, preferably between 25 and 35 nm.Advantageously, this antireflection layer includes at least one titaniumdioxide layer having a thickness of between 15 and 35 nm, preferablybetween 20 and 30 nm, and may also include another layer of a metaloxide that is deposited on said titanium dioxide layer and has athickness of between 5 and 15 nm, preferably between 6 and 10 nm. Thismetal oxide layer is preferably tin oxide (SnO₂) or silicon nitride(Si₃N₄).

With such features, the substrate according to the invention has asurface resistance not exceeding 1 Ω/□, preferably between 0.7 and 0.9Ω/□.

The substrate may be made of toughened or untoughened glass, or made ofplastic.

It will be advantageous to use such a substrate in an electromagneticshielding filter, applied for example to a display screen of the plasmadisplay type. This filter therefore comprises a substrate provided withthe multilayer of the invention, together with one or more functionalplastic sheets (for example with pigments or dyes) and/or anothertransparent substrate, optionally coated with an antireflection layer,so as to have the following optical properties:

-   -   a light transmission factor T_(L) of between 45 and 55%;    -   a purity of less than 10% in transmission;    -   a light reflection R_(L) of less than 5%, preferably less than        4%;    -   a predominantly violet-blue color in reflection with a purity of        less than 20%;    -   a predominantly blue color in transmission.

Other features and advantages of the invention will now be describedwith regard to the appended drawings, in which:

FIG. 1 illustrates a first embodiment of an electromagnetic shieldingfilter;

FIG. 2 illustrates a second embodiment of an electromagnetic shieldingfilter; and

FIG. 3 illustrates schematically the multilayer of the invention.

It should firstly be pointed out that the proportions relating to thevarious dimensions, especially thicknesses, of the elements of theinvention have not been drawn to scale in the figures so that they areeasier to read.

FIG. 1 illustrates a first example of an embodiment of the transparentstructure 1 intended to be joined to the front face of a plasma displayin order to form an optical and electromagnetic shielding filter.

The structure 1 comprises a first transparent substrate 10, which forexample is of the glass type but which could, as a variant, be made ofplastic, intended to be placed on the same side as the display, athin-film multilayer 20 according to the invention, which is placed onthe internal face of the substrate 10, facing the inside of thestructure, a second substrate 30 of the glass type, which is joined tothe first substrate, facing the multilayer 20, by means of a plasticfilm 40, such as a PVB film. This functional plastic film mayadvantageously include a mineral pigment or an organic dye so as tofilter the orange color of wavelength centered on 590 nm. The reader mayrefer for further details about the plastic film or alternativeembodiments of the structure to French patent application FR 03/04636.

The external faces of the substrates 10 and 30 to the outside of thestructure are preferably provided with an antireflection coating 50.

FIG. 2 illustrates a second example of an embodiment of the structure 1,which in this case comprises a substrate 10 one of the faces of which,intended to be on the opposite side from the observer, is provided withthe thin-film multilayer 20, and a substrate 60 made of plastic, such asPET, which is intended to be placed on the same side as the display andis joined to the substrate 10, facing the multilayer 20, by means of aplastic film 40, such as a PVB film, which may advantageouslyincorporate other functionalities as described above in the firstembodiment. The external face of the substrate 10, to the outside of thestructure, is preferably provided with an antireflection coating 50.

The invention therefore relates to the multilayer 20 deposited on asubstrate, such as the substrate 10. This multilayer includes threemetallic silver layers, Ag₁ being the layer closest to the substrate,Ag₂ being the central layer and Ag₃ being the one furthest away, thefunction of which is to reflect the electromagnetic waves having afrequency between 30 MHz and 1 GHz and infrared waves beyond 800 nm.

The multilayer includes, deposited alternately on the substrate, atitanium dioxide layer 21, a layer 22 of a metal oxide, consisting ofzinc oxide, one of the silver layers Ag₁, Ag₂ or Ag₃, and a layer 23 ofa sacrificial metal coating. Deposited on top of the sacrificial metallayer 23, which is deposited on the silver layer Ag₃ furthest from thesubstrate, is an antireflection layer 24 consisting of at least onemetal oxide.

The thickness of each of the silver layers Ag₁, Ag₂ and Ag₃ is between13 nm and 19 nm. The thicknesses e_(Ag1), e_(Ag2) and e_(Ag3) of therespective layers Ag₁, Ag₂ and Ag₃ may be identical or they may vary ina ratio of between 0.8 and 1.2 and are such thate_(Ag1)≦e_(Ag3)≦e_(Ag2). The imbalance in layer thicknesses ispreferential, so as to lower the light reflection while maintaining thesame surface resistance.

The titanium oxide layer 21 as sublayer for the silver layer Ag₁ closeto the substrate has a thickness of between 10 and 20 nm, preferablybetween 10 and 15 nm.

The titanium oxide layers 21 as sublayers for the other two silverlayers Ag₂ and Ag₃ have a thickness of between 35 and 55 nm, preferablybetween 40 and 50 nm.

The zinc oxide layer 22 preferably has a thickness of greater than 15nm, for example 16 or 18 nm.

The sacrificial metal layer 23 is of niobium, titanium or zirconium,preferably titanium, and has a thickness of at most 2 nm, for example1.5 nm.

This sacrificial metal layer makes it possible to protect the silveragainst oxidation, and to improve its resistivity. Although the presenceof titanium may degrade the light transmission, it does allow an evenlower surface resistance to be obtained, while maintaining asufficiently correct light transmission. The compromise to be foundbetween the optical properties of the filter and its shieldingproperties is provided by giving preference to shielding, while stillmaintaining good optical properties. Thus, with the sequence of theinvention based on three silver layers, the surface resistance drops to0.8 Ω/□, instead of 1.5 according to the prior art, which not only meetsClass A of European Standard EN 55022, dealing with what are called“consumer” products, but also Class B, dealing with special products ofthe home-cinema type.

The antireflection layer 24 for the silver layer Ag₃ remote from thesubstrate has a thickness of between 25 and 50 nm, preferably between 25and 35 nm. It comprises at least titanium dioxide with a thickness ofbetween 15 and 35 nm, preferably between 20 and 30 nm.

Advantageously, deposited on top of the titanium dioxide of thisantireflection layer is another metal oxide, of small thickness, between5 and 15 nm, and preferably between 6 and 10 nm. This metal oxide is,for example, tin oxide (SnO₂) or silica nitride (Si₃N₄)—which helps toimprove the purity of the colors in reflection and in transmission.

All the layers of the multilayer are deposited on the substrate by theknown technique of sputtering.

In the table below, we given give five examples (Ex 1 to Ex 5) of themultilayer 20 of the invention. Provided in the table are thethicknesses (in nm) of each layer and, for each multilayer joined to asubstrate 10, the values of the light transmission T_(L) (in %), thelight reflection R_(L) (in %), the purity in transmission p_(e) in T (in%), the purity in reflection p_(e) in R (in %), the dominant wavelengthsin transmission and in reflection, respectively λ_(d) in T and λ_(d) inR (in nm) and the surface resistance R_(surf) (in Ω/□).

These five examples make it possible to achieve suitable shielding lessthan 1 Ω/□.

In the case of examples 1, 2 and 5, the silver layers are the same andequal to 15 nm; the zinc oxide thicknesses are different, with athickness of less than 15 nm, exactly equal to 10 nm in the case ofexample 5. For each example, the thickness of the titanium dioxidelayers is fixed so as to optimize the optical properties of themultilayer.

The results show that, for examples 1 and 2 which have larger zinc oxidethicknesses than example 5 (from 6 to 8 nm and higher), the lighttransmission, contrary to what might be expected as regards the priorart, remains substantially the same and even slightly better in the caseof example 1 with a zinc oxide thickness of 18 nm, and the reflectionhas the advantage, in the case of examples 1 and 2, of being lower thanin the case of example 5, thereby making it possible for the display tobe illuminated less brightly and aggressively for the observer.

Examples 3 and 4 provide a comparison, with unequal thicknesses asregards the silver layers, with, in the case of example 4, anantireflection layer 25 based on SnO₂. It may be seen that the imbalancehas the advantage of reducing the light reflection but has the drawbackof increasing the purity in transmission and in reflection; the additionof the antireflection layer helps to overcome this drawback and thusobtain a purity in transmission equivalent or substantially equivalentto that of examples 1, 2 and 5, and to reduce the purity in reflectioncompared with that of example 3. Ex 1 Ex 2 Ex 3 Ex 4 Ex 5 TiO₂ 12 12 1212 13 ZnO 18 16 16 16 10 Ag₁ 15 15 13.5 13.5 15 Ti 1.5 1.5 1.5 1.5 1.6TiO₂ 43 43 43 43 48 ZnO 18 16 16 16 10 Ag₂ 15 15 16.5 16.5 15 Ti 1.5 1.51.5 1.5 1.5 TiO₂ 43 43 43 43 48 ZnO 18 16 16 16 10 Ag₃ 15 15 15 15 15 Ti1.5 1.5 1.5 1.5 1.5 TiO₂ 25 25 25 25 25 SnO₂ 0 0 0 7 0 T_(L) in % 62 6162 65 61 R_(L) in % 5.8 5.0 4.7 4.7 6 ρ_(e) in T (%) 5 6 9 6 5 λ_(d) inT (nm) 500 490 496 499 500 ρ_(e) in R (%) 30 20 50 40 30 λ_(d) in R (nm)−555 −560 −553 −547 −555 R_(surf) (Ω/□) 0.8 0.8 0.8 0.8 0.8

Thus, by controlling the deposition of the silver and dielectric layersand the thicknesses formulated according to the invention, and also bythe use of metal protection layers, the filter obtained with referenceto FIG. 1 or FIG. 2 has the following properties:

-   -   a surface resistance of less than 1 Ω/□;    -   a light transmission factor T_(L) of between 45 and 55%;    -   a purity in transmission of less than 10%;    -   a light reflection R_(L) of less than 5%, preferably less than        4%;    -   a predominantly violet-blue color in reflection with a purity of        less than 20%; and    -   a predominantly blue color in transmission.

The electromagnetic shielding filter using the substrate of theinvention may be applied to a display screen, in particular a plasmadisplay. It provides very good performance as regards shielding (thesurface resistance being less than 1 Ω/□), and it consequently blocksespecially infrared with a transmission at 900 nm that does not exceed1%. This filter also provides good visibility—a light transmissionfactor between 45 and 55%—and improves the contrast of the display.

1. A transparent substrate, especially made of glass, provided with athin-film multilayer (20) that includes three silver layers (Ag1, Ag2,Ag3) and comprises, alternately on the substrate, a titanium dioxidelayer (21), a metal oxide layer (22), one of the silver layers (Ag1,Ag2, Ag3) and a covering layer (23), characterized in that: the metaloxide is zinc oxide; the covering layer (23) is a sacrificial metal; andan antireflection layer (24) comprising at least one metal oxide isdeposited on the covering layer (23) for the silver layer (Ag3) furthestaway from the substrate.
 2. The substrate as claimed in claim 1,characterized in that the thickness of each of the silver layers (Ag₁,Ag₂, Ag₃) is between 13 nm and 19 nm.
 3. The substrate as claimed inclaim 2, characterized in that the thicknesses (e_(Ag1), e_(Ag2),e_(Ag3)) of the respective layers (Ag₁, Ag₂, Ag₃) are identical, or elsethey vary in a ratio of between 0.8 and 1.2 and are such thate_(Ag1)≦e_(Ag3)≦e_(Ag2).
 4. The substrate as claimed in claim 1,characterized in that the titanium dioxide layer (21) as sublayer forthe silver layer (Ag₁) closest to the substrate has a thickness ofbetween 10 and 20 nm, and the titanium oxide layers (21) as sublayersfor the other two silver layers (Ag₂, Ag₃) have a thickness of between35 and 55 nm.
 5. The substrate as claimed in claim 1, characterized inthat the zinc oxide layer (22) has a thickness of greater than 15 nm. 6.The substrate as claimed in claim 1, characterized in that thesacrificial metal layer (23) is of niobium (Nb), titanium (Ti) orzirconium (Zr).
 7. The substrate as claimed in claim 1, characterized inthat the sacrificial metal layer (23) has a thickness not exceeding 2mn.
 8. The substrate as claimed in claim 1, characterized in that theantireflection layer (24) has a thickness of between 25 and 50 nm,preferably between 25 and 35 nm.
 9. The substrate as claimed in claim 8,characterized in that the antireflection layer (24) includes at leastone titanium dioxide layer having a thickness of between 15 and 35 nm.10. The substrate as claimed in claim 9, characterized in that theantireflection layer (24) includes a titanium dioxide layer and anotherlayer of a metal oxide that is deposited on said titanium dioxide layerand has a thickness of between 5 and 15 nm.
 11. The substrate as claimedin claim 10, characterized in that the metal oxide layer of theantireflection layer (24) is tin oxide (SnO₂) or silica nitride (Si₃N₄).12. The substrate as claimed in claim 1, characterized in that it has asurface resistance not exceeding 1 Ω/□.
 13. The substrate as claimed inclaim 1, characterized in that it is made of toughened or untoughenedglass, or made of plastic.
 14. An electromagnetic shielding filtercomprising a substrate as claimed in claim 1, characterized in that ithas the following optical properties: a light transmission factor T_(L)of between 45 and 55%; a purity of less than 10% in transmission; alight reflection R_(L) of less than 5%; a predominantly violet-bluecolor in reflection with a purity of less than 20%; a predominantly bluecolor in transmission.
 15. A display screen of the plasma display typeincorporating, on its front face, at least one substrate or filter asclaimed in claim 14.